Embodiments of the present invention relate generally to multi-phase flow metering, and more specifically, to the use of antennas in measuring multi-phase fluid fractions.
A multi-phase fluid refers to a composition that includes at least two phases of material. For example, multi-phase fluids may include some combination of oil, water, and gas. In process industries, oil and gas industries and other such areas, it is often necessary to accurately measure fractions and flow rate of phases of the multi-phase fluid flowing inside a pipeline. With smaller and deeper oil/gas wells with higher water content becoming more common around the globe, there is an enhanced need for multi-phase flow measurement techniques.
Commercially available sensors for measuring fractions in fluids in the petroleum industry are based on a variety of principles (either a single technique or a combination of several techniques). For example, impedance sensors, capacitive and/or inductive sensors, dual-energy gamma sensors, venturi meters, and microwave sensors (attenuation/phase/resonance) have all been used. Currently, there are numerous microwave-based flow metering sensors available offering varying degrees of sensitivity, complexity and costs.
In current microwave based sensing systems, one or more sensors are excited over a range of frequencies to emit electromagnetic waves and the power of reflected and/or transmitted electromagnetic waves is measured over that frequency range. The resonant frequency of the system depends on the permittivity (dielectric constant) of the medium inside the pipe. A resonant mode can be characterized by the resonant frequency, Q factor and the amplitude of the peak in the transmitted or reflected electromagnetic waves. The shift in resonant frequency, or Q factor, or amplitude of the transmitted or reflected electromagnetic waves corresponding to a change in composition is used to estimate the phase fraction of the fluid. The resonant frequency and Q factor hold a proportional relationship with the dielectric constant. For instance, as the water content increases in the multi-phase fluid, the effective dielectric constant increases and a change in the resonant frequency, caused by the dielectric constant of the water, decreases. Similarly, if the water is lossy, e.g., due to salinity, that will result in a decrease in Q-factor of the reflected electromagnetic waves. Estimation of phase fractions using the resonant frequency method becomes difficult as the water content increases since identification of the peaks in the transmitted or reflected electromagnetic waves becomes a challenge.
Furthermore, some of the microwave-based metering sensors are intrusive to the flow inside the pipe. Consequently, these sensors may be exposed to the flow of the material inside the pipe, thereby increasing the possibility of damage to the sensors and necessitating frequent replacement. In other scenarios, the flow of the material may be measured by diverting the actual flow of material inside the pipe through an external flow circuit. In certain other scenarios, a flow separator or a flow mixer may be used.
Accordingly, there is an ongoing need to improve the performance of multi-phase flow metering in lossy media.